The present invention relates generally to a plasma processing apparatus and a method of performing various plasma processes. More specifically, the present invention concerns depositing a thin film of material on an object, such as a semiconductor wafer, through a plasma enhanced chemical vapor deposition process.
In the formation of integrated circuits, thin films of material are often deposited on the surface of a substrate, such as a semiconductor wafer. These thin films may be deposited to provide conductive and ohmic contacts in the circuits and between various devices of those circuits. For example, a thin film might be applied to the exposed surface of a contact or via hole on a semiconductor wafer, with the film passing between the insulative layers on the wafer. This would provide plugs of conductive material and allow interconnections across insulating layers. Those skilled in the art are aware of additional reasons for applying thin films on an object.
One well known process for depositing thin metal films is chemical vapor deposition (CVD). In this process, deposition occurs as a result of chemical reactions between various gasses. These gasses are pumped into a chamber that contains the wafer. The gasses subsequently react, and one or more of the reaction by-products form a film on the wafer. Any by-products that remain after the deposition are removed from the chamber. Films deposited by CVD tend to exhibit good step coverage. Step coverage indicates the ability of a material to consistently cover an underlying topography. For example, if a film is deposited onto a surface and into a trench recessed in that surface, it would be ideal if the thickness of the film on the surface was the same as the film thickness on the bottom and sides of the trench. In general, if the film thickness at the bottom of the trench is at least 50% of the thickness at the top of the trench, the step coverage is considered to be good. One of ordinary skill in the art can appreciate however, that the 50% benchmark is merely a general guideline and that specific benchmarks depend on the particular chemistry used for deposition. Traditional CVD processes often result in a step coverage of at least 50%. However, while CVD is useful in depositing films, many of the traditional CVD processes are thermal processes requiring high temperatures in order to obtain the necessary reactions. These high temperatures may adversely affect the devices comprising the integrated circuit.
One way to lower the reaction temperature is to ionize one or more of the component gasses used in the CVD process. Such a technique is generally referred to as plasma enhanced CVD (PECVD). Prior art teaches ionizing the gas in one of two places differentiated by their proximity to the chamber. One option is to ionize the gas locallyxe2x80x94creating a plasma within the chamber immediately above the wafer. This can be accomplished by providing electrodes within the chamber. Based on the frequency and power generated by the electrodes, an electric field is generated right over the wafer that ionizes the gasses and attracts the plasma to the wafer surface. The gas is introduced into the chamber through a xe2x80x9cshowerheadxe2x80x9dxe2x80x94a receptacle that defines a volume, receives a process gas into that volume, and allows the gas to disperse through a porous portion, such as a wall defining a plurality of perforations. In doing so, the showerhead serves to distribute the gas throughout the chamber in a more even distribution than would occur if the gas simply entered the chamber through a single hole. Oftentimes, the showerhead itself acts as one electrode, and the wafer support surfacexe2x80x94also known as the susceptorxe2x80x94serves as another electrode. Accordingly, the showerhead can be coupled to a radio frequency (RF) power source (or other power source) and the susceptor can be grounded. Conversely, the showerhead may be grounded while RF power is applied to the susceptor. Regardless of what components serve as electrodes, the resulting RF field induces a glow discharge, which transfers energy to the reactant gasses that have been introduced into the chamber. As a result, free electrons are created within the discharge region. These free electrons gain sufficient energy from the RF field so that when they collide with gas molecules, gas phase dissociation and ionization of the reactant gasses occurs. Energetic species, or free radicals, diffuse out of the plasma and are then absorbed onto the wafer. However, an unfortunate side effect of generating the plasma between the showerhead and the wafer is that heavy ions from the plasma, having an energy of up to 1.5 KeV per ion, impact the wafer and cause physical damage to the structures on the wafer. Moreover, the electrons freed by the RF field may also impact the wafer and alter the electrical properties of certain materials on the wafer, such as a gate oxide.
Another option that avoids this potential damage is to generate the plasma remotelyxe2x80x94away from the chamber. As demonstrated in U.S. Pat. No. 5,624,498, by Lee, et al., a plasma can be created by ionizing a gas used in the deposition process at a location removed from the chamber. The plasma is then passed through a conduit and is introduced into the chamber through the showerhead. Other gasses, whether they are ionized or not, enter the chamber from another entrance. Once inside the process area, the gasses interact with each other and, hopefully, produce the by-products needed to form the thin film on the wafer. Because the particles constituting the plasma lose their energy over time, generating the plasma a suitable distance away from the chamber results in the deposition materials impacting the wafer surface with less energy than deposition materials diffusing from locally generated plasma. Thus, there is less physical damage done to the wafer. In addition, the electrons recombine with ions and therefore create fewer changes in the electrical properties of the wafer. However, during the time taken to reach the chamber, the free radicals needed to deposit onto the wafer also stabilize, thereby slowing the deposition process. As a result, step coverage is poor and either the required deposition time is increased or the amount of gasses used must be increased.
Given these limitations, there is a need in the art for a plasma processing method that provides deposition superior to that of remote plasma-generation systems yet avoids the wafer damage caused by local plasma-generation systems.
Accordingly, the present invention provides a plasma-generation area for a plasma-processing system, wherein said area is located intermediate the conventional remote and local plasma-generating locales employed in the prior art. One exemplary embodiment of the present invention places this intermediate plasma generation area within a shell or volume defined by a showerhead located in the process area. Within the showerhead of this embodiment, a selection of gasses are ionized. Materials to be deposited are formed outside of the showerhead. This embodiment has the advantage of allowing the activated gas molecules to reach the substrate without the recombination that occurs in remote systems. The deposition materials will also be more uniformly distributed, which will allow for a more uniform film on the substrate. This and other exemplary embodiments also have the advantage of avoiding the damage caused by local plasma-generation systems. The present invention also comprises methods for achieving these advantages.